1. Field of the Invention
The present invention relates to video encoders, and more specifically to rate controlled predictive video encoders.
2. Discussion of the Related Art
Rate control is an essential function of most video encoders. This is due, in part, to the fact that while many encoders encode video as a variable bitrate stream, many applications require a constant bitrate stream. However, most video standards do not specify how to control the bit rate.
Conventional encoders control the output bitrate by varying the coarseness of the quantization step. Variation of the quantization scale directly affects the size of the output. Although coarse quantization significantly reduces the bitrate, it also reduces image quality. Accordingly, more advanced rate control algorithms are needed in order to distribute available bits in a manner that avoids visible artifacts in the displayed image.
Many rate control algorithms have been proposed, such as those disclosed ISO-IEC/JTC1/SC29/SG11, “MPEG2: Test Model 5,” Draft, April 1993 and Zhihai He and Sanjit K. Mitra, “Optimum Bit Allocation and Accurate Rate Control for Video Coding via p-domain Source Modeling,” IEEE Transactions on Circuits and System for Video Technology, October 2002. A look-ahead approach has also been introduced to rate control for stable rate control in case of noticeable change in the picture characteristic due to, for example, scene change. Such rate control schemes usually consist of two steps. The first step is to allocate bits to each picture in a given set of pictures to minimize the overall distortion. The second step is to find the quantization parameter that meets the number of bits allocated to encode the current picture. The rate control schemes identified above are used in the second step (i.e., selection of a quantization parameter). However, this two-step approach makes the rate control complicated.